It is well known to use photoluminescent storage phosphor screens (hereinafter referred to as a "phosphor screen") for various purposes, including computed radiography. Such phosphor screens may be created by applying a phosphor layer onto a substrate which may be formed of a polymeric material. The phosphor screens include materials capable of trapping electrons when exposed to ionizing radiation energy.
The phosphor screens typically include a thin, flexible substrate which can be coated with a layer of phosphor powder. A schematic representation of a typical phosphor screen 9 in shown in FIG. 1. In FIG. 1, a phosphor layer 10 is situated on top of a substrate 12. A protective layer 14 covers the top of the phosphor layer 10. Phosphor grains or particles 16 can be found throughout the phosphor layer 10. Such phosphor screens, when exposed to radiation photons, are capable of storing an image, or spatially varying energy pattern, by trapped electrons. The screens undergo a reversible alteration of the electronic state of the screen when they are exposed to the radiation photons. The state is reversed by mildly exposing the screen to infrared photons, which is accompanied by emission of more photons within the wavelength range of the visible spectrum. Thus, the phosphor screen can absorb the radiation pattern, store the information as trapped electrons, and later be read optically by converting the stored radiation pattern to a visible pattern.
Most phosphor screens include a phosphor composition which uses a base material such as strontium s sulfide (SrS) crystalline material. One such screen is available from Liberty Technologies, Inc. of Conshohocken, Pa. The crystalline material is doped with trace amounts of rare earth ions, for example, as in the Liberty Technologies' composition, cerium ions (Ce.sup.3+) and samarium ions (Sm.sup.3+). The strontium sulfide, when doped with the rare earth ions, generates new energy levels within the crystalline lattice. The function of the ions in the crystal lattice will now be described in further detail.
The ions consist of a nucleus of protons and neutrons, surrounded by outer electrons. The electrons surrounding the nucleus can only occupy certain energy levels which can each accommodate a fixed number of electrons. Electrons can undergo transition between levels if the levels are only partially filled. Transition of an electron from a lower energy level to a higher energy level requires an absorption of energy by the electron. Transition of an electron from a higher energy level to a lower energy level requires an emission of energy by the electron. With respect to the rare earth ions, the 4f level is only partially filled, but is surrounded by electrons in higher energy levels. As such, the electrons can undergo transition, for example, the 4f electrons can move to the higher 5d level. The energy difference between the 4f and the 5d levels is similar to visible light energy such that the 4f electrons can be excited to the 5d level by absorption of visible light. As a further example, the 5d electrons can move to the 4f level accompanied by the emission of light. These transitions are shown in FIG. 2 in which the nucleus N is shown with respect to the corresponding energy levels L.sub.1, L.sub.2, L.sub.3, L.sub.4, and L.sub.5. The energy E provided to the 4f level in the form of visible light causes the shift of an electron to the 5d level, and the emission of photons P causes the shift of an electron from the 5d to the 4f level.
When the rare earth ions are introduced within the crystalline lattice, the energy level configurations change due to interaction between the ions' electron energy levels with the electron energy levels of the strontium sulfide crystal. Further, the electrons of the rare earth ion energy levels may interact with each other. Examples of such interactions are shown in FIG. 3. As shown, when the crystal is exposed to ionizing radiation, electrons from the valence band are excited to the conduction band. The movement of the electron leaves behind a net positive charge, or "hole". The electron and hole are referred to as an "electron-hole pair". Electron-hole pairs are movable within the lattice, however, due to the potential barriers, the pair generally remains bound as it travels through the lattice. The bound pair is known as an "exciton".
Excitons are long-lived in strontium sulfide and can migrate through the lattice for some time before recombining and neutralizing each other. Such excitons preferentially recombine at distortions such as at the occurrence of a cerium ion within the lattice. The energy generated from the recombined pair is transferred to the cerium ion which results in excitation of the cerium ions' ground level 4f electron to a 5d level. Once in the 5d level, it can either move back to 4f, or tunnel to a neighboring samarium ion. The probability that this will happen increases with the number of available samarium ion sites near the cerium ions. Once the exchange of electrons takes place, the cerium ion (Ce.sup.3+) becomes Ce.sup.4+, and the samarium ion (Sm.sup.3+) becomes Sm.sup.2+. This process is referred to as "electron trapping". The cerium is the luminescent center, and the samarium is the "trap". By creating a population of trapped electrons in the phosphor screen, a latent image is created.
The trapping process is reversed by stimulating electrons trapped at Sm.sup.2+ sites with external energy as shown in FIG. 3. The energy to move the trapped electron to an excited state is about 1 eV which is equivalent to about a 1 .mu.m wavelength photon. The optical stimulation wavelength range for a strontium sulfide crystalline lattice doped with cerium and samarium is shown in FIG. 5 which shows the peak sensitivity at 1 .mu.m in the near-infrared (NIR) region.
Once in the excited state, the electron can tunnel back to its Ce.sup.4+ neighbor and drop its energy level to create luminescence, properly referred to as "photostimulated luminescence" or "PSL". The intensity of the PSL is directly proportional to the number of trapped electrons which is proportional to the amount of radiation energy absorbed by the phosphor screen.
In the absence of a neighboring samarium ion, the cerium electron from the recombined excited pair in the 5d level would likely move back to the 4f level, generating visible photons. This process is known as prompt luminescence or "fluorescence". The luminescence spectrum for the cerium ion is shown in FIG. 4.
The rate at which electron trapping occurs depends upon the rate at which the various trapping steps take place, including exciton generation; exciton recombination at cerium ion sites; transition between the 4f and 5d energy levels of the cerium ion; tunnelling between cerium and samarium ions; and electron movement from the Sm.sup.2+ excited state to the ground state. The rate at which excitons are generated, and therefore, the number of excitons, is proportional to the rate of radiation energy absorption, i.e., the dose rate. Most of the trapping steps occur very fast in comparison with the rates of exciton generation and recombination. As such, the rate equations which best express the rate of electron trapping are as follows: ##EQU1## wherein, f is the exciton generation rate, n.sub.e is the number of excitons generated, N is the number of available trapping sites, n is the number of trapped electrons and A is the transition coefficient for trapping, i.e., A provides the probability with which trapping may occur. Equations (I) and (II) are solved to yield Equation III below for the build up of trapped electrons: EQU n.sub.(t) =N[1-exp(-ft/N)] (III)
wherein ft=.gamma.D, D is the radiation dose and .gamma. is the proportionality factor. By plotting the number of trapped electrons, n, against exposure time (dose, D), it can be seen that the number of trapped electrons increases linearly until the traps have been saturated as shown in FIG. 6.
Once the latent image is stored as trapped electrons according to the above described occurrences, the image must be "read" by stimulating the trapped electrons with NIR photons to create visible luminescence and to render an image which can be observed or electronically recorded. The rate at which this occurs is dependent upon the rates of absorption of photons by the Sm.sup.2+ ions and excitation of trapped electrons; tunnelling of electrons from the Sm.sup.2+ excited state to a neighboring Ce.sup.4+ ion; and movement from the Ce.sup.3+ ion 5d to 4f level and emission of luminescence. The rate of tunnelling and movement and emission are very fast in comparison with the rate of absorption of photons and excitation of trapped electrons. Therefore, the rate at which luminescence is emitted, i.e., luminescence intensity, is approximately equal to the rate at which trapped electrons are stimulated with photons as described in Equation (IV): ##EQU2## wherein, .sigma. is the infrared photon capture efficiency of the Sm.sup.2+ ions, and I is the infrared intensity absorbed by the phosphor, and .sigma.I is the time constant for emptying the traps.
Once the phosphor screen absorbs energy, the visible luminescence pattern, i.e. the image, must be converted to a permanent, easily viewable format. As shown in FIG. 7, a phosphor screen 9 is scanned with a laser beam 18 and only a small volume of the phosphor layer is photostimulated at any given time. The remaining areas are left undisturbed. The scanning mirror 20 is digitally controlled to the precise laser beam position on the screen. The PSL intensity from the small phosphor area is then measured with a light sensor, for example, a photomultiplier tube 22 which converts the light into electrical current. The current is converted to voltage and digitized. The digital voltage value is stored in computer memory as a function of the x-y coordinates on the screen 9. The process of reading each small portion is repeated across the entire screen 9.
The scanning process is very dependent upon the scanning speed. The time constant, .sigma.I of the phosphor, as described above, should be much faster than the scanning speed. After the laser leaves a given small area, known as a "pixel", the photostimulated luminescence should drop to a negligible level to avoid creating an afterglow or image haziness known as "lag." Lag reduces image quality which creates problems particularly with high resolution scanning. During high resolution scanning, the number of pixels is increased, requiring a longer time to complete scanning. If the phosphor does not have a sufficiently high response time constant, a high resolution scan will take unacceptably long to complete, which limits the practical usefulness of such a screen. As such, there is a need in the art for a phosphor composition having a high response time constant and which reduces lag to facilitate the ability of a phosphor screen to be scanned in high resolution scanning processes. For high resolution scanning, the preferred speed is about 1 .mu.s/pixel.
In preparing a phosphor screen, the response time constant of the phosphor, or speed with which the screen will respond as well as the sensitivity of the phosphor; and thereby the scanning speed and the image quality, are influenced by a variety of factors including the choice of the substrate, the thickness of the layer and the nature of the phosphor layer.
As a beam passes deeper through the phosphor layer, the scattering events increase. Further, the phosphor particles have the ability to absorb and to scatter the beam such that the particles can also affect the efficiency with which visible luminescence created in the phosphor layer is able to escape from the phosphor layer to form an image, i.e., it effects exposure time. The longer it takes for the image to appear, the longer the exposure time of the screen. Phosphor layers having increased thickness allow for deeper penetration of the beam and an increased generation of luminescence such that less radiation is required to generate an image. However, deep penetration also increases scattering and can contribute to a reduction in image quality. The substrate can also effect image quality. Reflection from the substrate provides additional scattering and stimulation of phosphor material away from the point of incidence of the beam on the phosphor layer such that image quality is reduced and lag is created. There is a need in the art for a phosphor screen which has a phosphor layer and substrate which reduce lag and provide good image quality at high resolution and under fast scanning speeds.
One prior art process of providing a phosphor screen for use in radiography is described in U.S. Pat. No. 4,855,603. A phosphor composition containing a base material is formed from an alkaline earth metal sulfide, specifically strontium sulfide, a first dopant which is samarium to provide electron trapping sites, and a second dopant which may be cerium oxide, cerium fluoride, cerium chloride and cerium sulfide. The composition may also include a lithium fluoride fusible salt and a barium sulfate component to provide an improvement to emission efficiency. This patent further teaches that cesium halide may be provided to the mixture to improve the light output intensity, i.e., the emission efficiency.
The phosphor screen is made by grinding and homogenizing the mixture of components and heating the components in a nitrogen atmosphere to between 950-1300.degree. C. for 30 minutes to an hour to form a fused mass. The mass is cooled and ground to a powder having particle sizes ranging from 5 to 100 microns. The particle size distribution is wide and the majority of the preferred particles selected for further processing are in the range of about 28-34 microns. Those collected fine particles less than about 10 microns are removed as "scrap," and reprocessed in the composition to achieve higher particle sizes after fusing.
After grinding, some form of etching of the crystals must be done to remove metal deposited on the crystals from grinding. One part of cesium iodide per 100 parts of the total composition is added, and the composition is reheated below fusing temperature at about 700.degree. C. for about 10 to about 60 minutes to regenerate the crystals and repair crystal damage. The material is then cooled and mixed with an acrylic, polyethylene or other organic polymer binder and applied as a coating of between 100 and 500 microns in thickness on a substrate. The substrate may be transparent or opaque and formed of clear plastic, aluminum oxide, glass, paper or other solid substance.
In producing a phosphor screen according to the prior art, in order to generate visible images from stimulation by infrared light, the particles are typically ground to a particle size of from 10 to 100 microns absent some form of particle evaporation or sputtering technique. Typical grinding processes used for forming the phosphor compositions used in phosphor screen formation include use of ball mills which use metallic or ceramic balls, e.g., alumina balls. Use of ball mills and other various mechanical grinding apparatus in prior art processes creates substantial contamination of the phosphorcomposition and damage to phosphor crystals.
Other phosphor screens such as that of U.S. Pat. No. 5,378,897 include a stimulable, divalent europium-activated barium fluorohalide phosphor coated on a support which may also include a metal oxide capable of reflecting stimulating rays. The screen is formed by dispersing the reflective metal oxide in a binder and solvent, and applying it evenly to the surface of a substrate. The europium-based phosphor is then combined with a binder and dispersed in a solvent. The dispersion is applied to a second releasable substrate and compressed on the reflective layer. The second releasable substrate is removed leaving the phosphor layer on top of the reflective layer.
Prior art phosphor screens, such as those described above are suitable for low speed usage. However, to achieve high resolution images, the number of pixels to be read must be increased. Increasing the number of pixels also increases the amount of time necessary to read the total number of pixels. However, an increase in scanning time to read a high number of pixels, particularly for industrial applications, is undesirable, unless the scanning speed is also significantly increased. Most radiography applications require as fast a reading time as possible. In order to increase the number of pixels which need to be scanned, and to maintain or improve industry acceptable scanning times, the scanning speed must necessarily also be increased, preferably about one-hundred-fold from scanning speeds used for prior art, low-resolution phosphor screens. However, when scanning speed is increased to such a high degree, prior art phosphor screens exhibit a significant reduction in resolution and sensitivity, and image lag is created. The lag, as described above, encountered with prior art phosphor screens is manifested by a hazy or fuzzy image when reading at fast scanning speeds. The hazy images are difficult to read when viewing the image initially recorded on the screen.
There is a need in the art for an improved phosphor screen which can be read at very fast scanning speeds of about 5 .mu.s/pixel, or as fast as 1 .mu.s/pixel, but which does not show significant lag when read by illuminating the recorded image with stimulating light.